It is known that, in order to start a turbine engine, use is made of a starter that delivers a driving torque to the shaft of said turbine engine. A driving torque such as this is intended to overcome all of the resistive torques imposed on said shaft and which are chiefly the result of the aerodynamic drag of the rotating parts, of the mechanical friction of the contacting parts and of losses by viscous friction between the contacting parts immersed in the fluid of at least one lubricating and/or heat-regulating hydraulic circuit associated with said turbine engine. The starter gradually accelerates the turbine engine and, when a preset rotational speed is reached, injection and ignition of fuel into/in the combustion chamber of the turbine engine are initiated. Then at another preset rotational speed, the action of the starter is halted and the turbine engine continues to accelerate up to its idling speed by virtue of the combustion of said fuel.
Known starters may be of the pneumatic type (in which a turbine is supplied with compressed air), but they are increasingly of an electrical type. In the latter instance, these are generally synchronous machines operating as motors during the turbine engine starting phase and operating as generators when the aircraft is in flight. In that which follows, only starters of an electrical type will be considered.
It will be readily understood that, in cold weather (for example when the ambient temperature and the temperature of the fluid in the hydraulic circuit associated with the turbine engine is below −20° C.) said fluid sets solid, thus greatly increasing the resistive torque that the electric starter has to overcome in order to start said turbine engine. Of course, the electric starter has to be able to perform such starting operations in cold weather, and this means that its power is rated for that. However, it must be pointed out that cold-weather starts are relatively infrequent events in the life of an aircraft, which means that it becomes necessary to use either starters that are excessively powerful and therefore heavy for most starting sequences, or procedures whereby the turbine engine hydraulic circuit is heated using external heat sources.
The disadvantage of an aircraft electric starter having excessive mass and excessive power is further exacerbated by the fact that, for airplane system electrical architecture reasons, there are two electric starters per turbine engine. This way, said turbine engine can be started even in the event of failure of one of the electric starters.
Furthermore, it should be noted that the viscous resistive torque developed by a turbine engine while it is being started in cold weather is difficult to predict with any accuracy. It is dependent on the precise temperature of the turbine engine fluid at the time of starting, on the quality and properties of this fluid, on the relative deformation of the contacting mechanical parts which is associated with the ambient temperature, and on other parameters that are difficult to predict. This is why the starter is rated on the basis of resistive torque curves that incorporate a design margin. Nonetheless, this precaution will not guarantee that the power of the starter will cover all cold weather starting scenarios which the aircraft is liable to encounter. The user has therefore to follow special procedures before attempting to start the engine in cold weather, which procedures prove detrimental to aircraft availability and operability and are unable to prevent failed attempts at starting turbine engines.
In order to remedy these disadvantages, document GB-A-2 260 577 describes a method for starting up a turbine engine using an electric starter, said turbine engine being provided with at least one hydraulic fluid circuit for lubricating at least certain parts of said turbine engine, according to which method, when said turbine engine is being started in cold weather, at least some of the heat energy generated by said electric starter is transferred to the hydraulic fluid of the turbine engine lubricating circuit.
Thus, the hydraulic fluid (oil) of the turbine engine lubricating circuit is heated up by said electric starter at the time of a cold start, such that the viscosity of said hydraulic fluid is reduced, thus reducing the resistive torque generated by this fluid. This results in shorter starting times, smaller starters, and lower risks of aborted starts.